70 SB Idle Oil Pressure Too Low for PV?

[Pat M.]

Hello again. At warm idle, the oil pressure of my 70 350/300 is quite low, between the 0 and 17lb marks on the gauge, yet closer to 0. The dipstick shows the oil level to be "full," although I do have very minor oil pan leaks that will be fixed shortly.

The car runs excellent, and just a light touch of the accelerator quickly brings the pressure to within PV specs of 35lbs. Moderate road speeds also register PV normal readings of 35lbs or higher.

Yet the PV manual states that non high-performance engines MAY have "a warm idle (pressure) of 20 pounds," and that "readings on a fully warm engine near either extreme of (the) oil gauge travel... constitute(s) a test failure."

Why is my oil pressure so low at warm idle? Will this, as the manual states, constitute a failure of PV? If so, what should I do to remedy this?

As always, thanks for your learned advice, Patrick Moresi #45581

[Duke W.]

Re: 70 SB Idle Oil Pressure Too Low for PV?

Most year AMA specs call for 40-45 psi hot oil pressure at 2000 RPM on base engines, and it should not increase beyond this range at higher revs. Typical slight oil leaks from non-pressurized gaskets have absolutely NO effect on oil pressure.

GM does not specify idle oil pressure AFAIK, so one must rely on anecdotal evidence. As you have observed, idle oil pressure is very senstive to idle RPM, so increasing RPM to the high end of the acceptable range for your engine will likely increase oil pressure.

While at the convention you should poll other base engine owners for their oil pressure @ idle speed, and whether it is a manual trans in neutral or auto trans in Drive. Like "20 psi at 500 PRM idling in neutral, manual trans". Or "15 psi @ 450 idling in Drive, auto trans".

For most pre-emission base SBs, GM's recommended idle speed is about 500 in neutral and somewhat lower in Drive with P/G. These recommendations should be in the AMA specs for each year (and they vary somewhat by model year), so everyone should have this document, which is available complimentary from GM (see archives).

Emission controlled engines typically have slightly higher specified idle speeds than their non-emission controlled antecedents.

Also in dash oil pressure gages are not laboratory precision instruments, especially electrical gages, so it would be a good idea for you to check idle oil pressure with a known accurate test gage to determine if the vehicle oil pressure gage system is reasonably accurate.

Duke

[Pat M.]

Thanks Duke. I should have added that ...

My car has a manual transmission, and that I was speaking of the oil pressure reading when the 4-speed was in neutral, and was operating at the (PV manual) specified warm curb idle of 700 rpm. Slight rpm increases, up to 2000 and slightly above, increase the oil pressure to within PV specs (35-40psi). But, as stated above, my SB warm idle oil pressure does not correspond with the PV manual specs of at least 20psi.

Also, as I'm sure you know, my original 70 gauge is mechanical, with an oil line, and not electrical.

[Mark #28455]

Quick fix for PV

You may want to try the diesel 15W40 and see if the cold pressure is any better. It may help.
Mark

[Duke W.]

Re: Thanks Duke. I should have added that ...

That does seem low for 700 RPM. Consider that mid sixties 250/300 HP 327s are speced at about 500 in neutral with manual trans, and I think they develop at least about 15 psi at idle.

Old Bourdon tube pressure gages can go out of cal, too, so you should still check it against a good test gage.

The engine only needs about 5-10 psi at idle, but most engines develop more at idle. I do think that GM reduced idle oil pressure on some engines, but not until a little later when CAFE went into effect.

Duke

[george romano]

Re: Thanks Duke. I should have added that ...

Pat-
My 1972 base engine 350 4 speed lists idle RPM at 850 if I remember correctly.
Can't check the underhood label right now, but I can check for you this afternoon. Might check yours, you could have it set too low.

George

[Pat M.]

Thanks for the input guys ...

Duke - I think the idle oil pressure is within the required minimum you described, so I'm not concerned about the engine. My only concern is if the PV judge thinks it's too low for PV specs, and fails the car because of it. I may double check the gauge as you suggest, and I may try a heavier oil as Mark and another member suggested.

George - my underhood label does list a higher idle of 800 rpm, but as I recall the PV manual specifically states that it's 700, and that in case of any conflict between the label and the manual, the manual controls. Maybe I'll try to bump up the idle just a hair above 700 as well, and see if all of these tweaks get me closer to 20psi at idle...

[Duke W.]

Re: Thanks for the input guys ...

It's a judges individual call, but I will accept GM recommendations and specs over any NCRS JG. That's why it's a good idea to bring the AMA specs to judging events.

Keep in mind that on emission controlled engines the idle speed is PART of the emission control system, so it is essentially a "legally binding" number, and the idle speed on the emission sticker (or AMA specs) certainly is more authoritative than whatever the JG says. A good judge should know this and take steps to get the JG corrected if it lists something other than the GM spec.

In California field emission tests, the idle speed must be within the manufacturer's recommended range or you bust the test, period. If there is no tolerance or range listed on the emission label, a plus or minus 100 RPM deviation is allowed or your car fails, even if the tailpipe numbers are squeaky clean!

Duke